1. Field of the Invention
The present invention relates to a magnetic disk apparatus used for recording/reproducing information and in particular, to a magnetic disk apparatus drive method when the magnetic disk apparatus is mounted on a vehicle such as an automobile where strict environmental conditions including operation temperature are required.
2. Description of the Related Art
The start torque required for starting a spindle motor is increasing as the number of magnetic disks increases in a magnetic disk apparatus and as the contact start/stop (CSS) method is employed. Japanese Patent Publication 2796333 diskloses a conventional technique for satisfying this requirement. According to this Publication, the spindle motor has a 3-phase Y-connected coil and a pulling line is provided in each of the coil phases, so that the number of windings of a stator is switched from one to another for controlling the motor. More specifically, in order to obtain a large torque required upon start, all the regions of the n windings are used upon start and during a process to reach a stationary rotation, the pulling line is used to switch to a partial region of the stator windings.
Moreover, the specification of U.S. Pat. No. 5,471,353 diskloses a technique for switching the drive method from one to another during a process to motor stationary rotation in a disk apparatus having a plurality of motor drive methods. According to this specification, the spindle motor has two or more coil windings for driving the spindle motor in unipolar mode and bipolar mode. More specifically, upon starting a disk drive, the bipolar mode is used and during the stationary rotation, the unipolar mode is used. Thus, upon starting the motor, it is possible to use the bipolar mode having a low current and a large torque. That is, by employing the bipolar drive method, it is possible to obtain a large torque upon start and in the process of rising, the mode is switched to the unipolar drive method, thereby enabling stationary rotation while suppressing the counter electromotive force.
It is assumed that a magnetic disk apparatus used as an external storage device is normally mounted on a personal computer and the temperature range for assuring operation is specified. For example, the conventional magnetic disk apparatus can work within a temperature range of 5 to 55 degrees C.
Recently, the magnetic disk apparatus has begun to be used in a portable device other than the personal computer and its use range is increasing. For this, the temperature range is also increased as compared to the conventional condition. For example, in a navigation system mounted on an automobile, the temperature condition is very strict and the magnetic disk apparatus has an operation temperature range of—30 to 85 degrees C.
On the other hand, the spindle motor for rotating a magnetic disk has conventionally used a ball bearing but now a structure using fluid dynamic bearing (FDB) is also used in practice in accordance with reduction in size and thickness of the apparatus. When rotating the motor having these bearings, grease used for the bearings has oil viscosity depending on the temperature and accordingly, the rotation load has a temperature characteristic. Especially when the temperature is low, the load increases and a large torque is required as compared to the normal temperature condition as the normal use state.
The spindle motor of the magnetic disk apparatus is a DC motor. In order to cover the aforementioned load fluctuation range, the motor should have such a torque characteristic that a counter electromotive force constant (equivalent to a torque constant) is lowered for the torque required at a low temperature, thereby assuring a large voltage margin (difference generated between the motor torque characteristic and the load characteristic) (which will be detailed later). Since this means that the torque constant is lowered, the motor drive current increases, resulting in a motor having a large power consumption. A portable device requires a reduced power consumption and the increase of the motor drive current is a problem to be solved.
Moreover, Japanese Patent Publication 2796333 shown as a conventional technique diskloses how to assure a large torque required upon motor start and a rotation margin during a stationary rotation but does not consider the torque generation required during a stationary rotation at a low temperature or reduced power consumption (drive current reduction) at a normal temperature. Furthermore, the aforementioned US patent specification diskloses an apparatus having a plurality of motor drive methods so as to employ the bipolar drive method upon start and the unipolar drive method to be switched during a start process and does not solve the problem concerning the stationary rotation at a low temperature.